The sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce audio signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be helped by the use of conventional hearing aids that amplify sound so that audio signals reach the cochlea and the hair cells. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensorineural hearing loss, on the other hand, is due to the absence or the destruction of the hair cells in the cochlea which are needed to transduce audio signals into auditory nerve impulses. Thus, many people who suffer from severe to profound sensorineural hearing loss are unable to derive any benefit from conventional hearing aid systems.
To overcome sensorineural hearing loss, numerous cochlear implant systems—or cochlear prosthesis—have been developed. Cochlear implant systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function. To facilitate direct stimulation of the auditory nerve fibers, an array of electrodes may be implanted in the cochlea. The electrodes form a number of stimulation channels through which electrical stimulation pulses may be applied directly to auditory nerves within the cochlea.
Hence, an audio signal may be presented to a patient by processing and translating the audio signal into a number of electrical stimulation pulses. The stimulation pulses may then be applied directly to auditory nerves within the cochlea via one or more of the stimulation channels. However, in traditional cochlear implants, important information from an audio signal can be lost during translation of the audio signal into electrical stimulation pulses. Such losses during translation may make it difficult for cochlear implant patients to enjoy the full advantages of unmitigated hearing. For example, losses during translation may make it difficult for cochlear implant patients to identify a speaker, understand speech, and appreciate the nuances of musical sound.
Studies suggest, however, that many advantages of unmitigated hearing can be obtained by hearing impaired individuals with enough residual hearing to clearly perceive, either naturally or with the aid of an amplification device, pitch and other fine structure information in the lower frequency ranges of audible sound. These studies, for example, show that clear perception of such fine structure information may improve performance in the above mentioned skills of speaker identification, speech recognition, and musical perception. Unfortunately, fine structure information is often lost, distorted, or otherwise adversely affected by traditional cochlear implants.